1. Field of the Invention
The present invention relates to a technology used to derive weight vectors, and it particularly relates to a method for deriving weight vectors to be used at the time of transmitting signals from a plurality of antennas, and a transmitting apparatus and a communication system utilizing said method.
2. Description of the Related Art
One of techniques to realize a higher quality and a higher data transmission rate in a wireless communication system is a MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of channels corresponding respectively to the antennas are set. Accordingly, channels up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to achieve a high data transmission rate. Of such MIMO systems, a MIMO eigenmode system can increase the channel capacity. In the MIMO eigenmode system, derived is a channel matrix (hereinafter referred to as “H matrix”) which is generated from values of channel characteristics between a plurality of antennas provided in the transmitting apparatus and those in the receiving apparatus. Then, eigenbeams corresponding to orthogonal channels the number of which is equal to the rank of an H matrix are formed in the MIMO eigenmode system. In so doing, the eigenbeams corresponding respectively to the orthogonal channels are formed.
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme is effective in countermeasuring the delay path. Also, when the MIMO eigenmode system is combined with the OFDM modulation scheme, the eigenbeam is formed on a subcarrier-by-subcarrier basis. In general, an eigenvector derived as a result of an eigenvalue operation per subcarrier does not have continuity among subcarriers. When the transmission is performed using weights having less continuity between the subcarriers, namely, in the frequency domain, there are cases where an impulse response of an equivalent channel observed at a receiving side has a delay spread.
On the other hand, when there is no transmission directivity, the estimation of a channel having the delay spread is done satisfactorily in the time domain. This corresponds to improving an SNR (Signal-to-Noise Ratio) by restricting an estimation interval within a guard interval if the impulse response of channels outside the guard interval gets smaller. Nevertheless, the impulse response in the MIMO eigenmode system as described above has a delay spread beyond the guard interval. Then, it is difficult to perform the estimation in the time domain, so that it is important to maintain the continuity of weights in the frequency domain.
The receiving apparatus generally estimates the channel characteristics from received signals, and carries out demodulation per subcarrier based on the estimated channel characteristics. Here, the receiving apparatus reduces the effect of noise contained in the estimated characteristics in order to improve the receiving characteristics of the signals. For example, the receiving apparatus performs smoothing processing in the time domain or smoothing processing in the frequency domain. However, as described above, the correlation in the signals in the frequency domain gets smaller in the MIMO eigenmode system. For such signals, if the smoothing processing in the frequency domain is performed, the orthogonal channel will not be formed. Accordingly, a receiving apparatus compatible with the MIMO eigenmode system cannot perform the smoothing processing in the frequency domain and therefore cannot reduce the effect of noise.